1. Field of the Invention
This invention relates to a photosensor array which can be used for deriving a light signal in photoelectric transducers for treating image informations such as facsimile transmitter and receiver, letter reading apparatuses and the like.
2. Description of the Prior Art
Heretofore, as photoelectric transducing means constituting image reading portions such as digital copiers, facsimile and the like, there have been generally used one dimensional photosensor arrays of a silicon photodiode type using crystalline silicon. However, since the size of silicon single crystal which can be produced is limited, the size of the photosensor can not be enlarged as much as might be desired. Further, from the standpoint of the processing accuracy, the length of the photosensor is limited and the manufacturing yield of the photosensor is disadvantageously low.
Therefore, when the width of an original to be read is large, for example, 210 mm, a lens system is used to form the reduced size image on a photosensor for reading. When such an optical system for reduction is used, it is difficult to miniaturize a light receiving portion, and moreover, it is inevitable to make the area of each picture element of the photosensor smaller to keep a high resolution. As a result, in order to obtain sufficient signal currents, a large quantity of light is required, and therefore, the photosensor array as mentioned above is now used for a low speed type apparatus where the reading time is long, or a reading apparatus where a high resolution is not required. From the standpoint of the driving of photosensors, in the above-mentioned diode type photosensor the photocurrent is so small that matrix-driving is difficult and there should be used a direct driving system where the sensor and integrated circuitry correspond to each other in such a manner as "one to one". Consequently, many driving integrated circuitry are inevitably required and, thereby, the photosensor array becomes expensive.
On the contrary, there have been recently proposed photoconductive type photosensors using amorphous silicon (a-Si). The photosensor is fabricated by forming an a-Si thin layer on a substrate according to vacuum deposition methods so that photosensor arrays of a large area or a large length can be easily produced. Thus, by means of photosensor arrays utilizing a-Si, the reading can be made at the same size even if the width of the original is large. As a result, the apparatus can be easily made smaller. In addition, in the case of photosensors of this type there can be obtained a photocurrent of about 100 times that in the case of photosensors of diode type so that matrix driving is possible resulting in a decrease in the number of integrated circuits used and in a low cost for photosensor arrays.
An example of conventional photoconductive type photosensor arrays is shown in FIGS. 1A, 1B and 1C. FIG. 1A is a schematic partial plan view, FIG. 1B is a schematic cross sectional view taken along a line X-X', FIG. 1C is a schematic cross sectional view taken along a line Y-Y'. In FIG. 1, 11 is a substrate, 12 a photoconductive layer as a photoelectric converting portion, 13 a common electrode layer, and 14 an individual electrode layer. In a photosensor array of such a structure as mentioned above, in the case that reading is effected by irradiating with image information signal light from the side of substrate 11, if light other than the image information signal light comes to photoconductive layer 12, it causes noise which interferes with the correct reading of the image formation, which noise appears in the electric signal output by the photosensor array. Further, in the case of the prior art photosensor array as mentioned above, photoconductive layer 12, common electrode layer 13 and individual electrode layer 14 are not electrically shielded, and therefore, external electric noise is liable to come in. As a result, the reading electric signal does not exactly correspond to the image information so that the resolution of reading is disadvantageously lowered.
For matrix driving of a photoconductive type photosensor array, there may be employed a multilayer wiring system where connecting electrodes of a driving circuit are arranged above electrodes of a photosensor with an insulating layer intervening therebetween and with the connecting electrodes and the photosensor electrodes electrically coupled at the portions at which the electrodes cross each other.
On the other hand, it is desirable to attach an insulating layer onto a photoconductive layer of a photoelectric converting portion of a photosensor.
Methods for fabricating such prior art photoconductive type photosensor array of matrix driving type are relatively complicated.